1. Field of the Invention
This invention relates to a semiconductor device and more particularly to a power switching device in the form of an insulated gate transistor with improved speed of turn-off.
2. Discussion of the Prior Art
An insulated gate transistor comprises four layers or regions of alternate conductivity-type semiconductor material disposed between an emitter electrode and a collector electrode. A gate electrode is positioned adjacent the collector electrode but is insulated from the body of semiconductor material. The two intermediate regions extend to the surface of the semiconductor body underneath the insulated gate and the intermediate layer closer to this surface extends to the collector electrode. When an appropriate potential is applied to the gate electrode, the electric field which is created serves to drive away majority carriers from and attract minority carriers to a nearby zone of that intermediate region thus creating an inversion channel through which charge carriers can pass between the adjacent regions. The device is thus switched on and current flows between the emitter and collector electrodes.
This insulated gate transistor is however slow to turn-off when the gate potential is removed. This is because at turn-off, a large number of charge carriers are present in the base region (the region adjacent the emitter region) and the current decays as these disperse. The turn-off speed can be improved by providing so-called lifetime killers or recombination centres in the base region. Such recombination centres can be introduced either by doping with interstitial impurities (e.g. gold) or by radiation damage but each of these techniques is expensive. Further, the provision of recombination centres introduces two other problems, firstly that the device is leaky (i.e. a relatively high leakage current flows between collector and emitter when the device is "off"), and secondly that the device exhibits a relatively high onstate voltage drop owing to the disrupted lattice. Moreover, even with recombination centres, the turn-off is still relatively slow.